Method of heat treating piano string frames



S. WEBER Dec 26, 1933.

INVENTOR film/6y Wh Patented Dec. 26, 1933 UNITED STATES PATENT OFFICE METHOD OF HEAT TREATING PIANO STRING FRAMES Stanley Weber, Long Island City, N. Y., assignor to Steinway 8; Sons, New York, N. Y., a corporation of New York 1 Claim. (01. 148-10) This invention relates to the method of making piano string frames. In the past many attempts have been made to produce a rib, over which the piano strings pass, which will be very hard and yet at the same time in no way affect the quality of the tone produced by the vibration of the strings. One of such attempts consisted in case hardening the entire casting, but during this process distortion and often rupture of the-casting occurred, rendering it useless for fine musical instruments. Furthermore, the entire casting, if hardened, would adversely affect the tones in the piano. Another attempt involved the chilling of the desired part, such as the rib, but such chilling requires machining to the exact dimensions for good piano practice, and accordingly this method was found to be too diflicult and too costly. An-

other attempt consisted in inserting studs or hard steel ribs into the cast iron, but the objection to this was the loss and dissipation of vibrational energy, and consequent deterioration of the tone quality. It is apparent, therefore, that a piano string supporting rib of desired ridge hardness is highly desirable, if it can be economically and simply produced or made.

It is an object of the invention to provide a method of producing a piano string frame or plate which is free from all of the various ob- Jections inherent in the older processes cited.

In the method of manufacturing piano plates or string frames, an unusually large proportion of charcoal or soft cast-iron, usually as much as fifty percent, is used. The castings formed are allowed to cool in the mold during a period of eight hours during which time the temperature drops from approximately 1600 C. down to 200 C. Upon analysis of the final casting it has been found that a large proportion, about three percent, in most cases from two to four percent, of carbon has separated out as graphltic carbon. This type of casting is commonly known as grey cast-iron. l ['he ribs of such frames were machined and the piano strings when passed, under tension, over the ridges of said ribs, would soon cut into the ridges of the ribs, and in consequence the tonal qualities would be detrimentally influenced.

This invention applies, for example. to an improved method of producing a piano plate in which the surfaces of the ridge of the rib adapted to contact with the strings are alone heat treated. In this method of localized treatment through heating .of the cast iron ridge of the rib of the plate bar or string-frame,- the ridge is heated to a temperature high enough to bring about a resolution of the carbon, graphitic, or combined, or both, in the metal of the ridge, and thereupon, this ridge is allowed to cool comparatively rapidly. The method described has application to cast iron parts other than; piano plate bars.

To describe the preferred method of the invention more in detail, as it relates to a piano plate bar, I proceed as follows: After the rib portion of the frame has been machined generally to the desired finish and dimension, the ridge portion of the rib upon which the strings are intended to rest, is subjected to the action of heat derived from an electric arc, which is played along the, ridge surface, using an electrode of either copper, carbon, or other suitable material, and employing an electric current of high amperage at'a current density of about one thousand to two thousand amperes per square inch. The electrode is passed over the edge of the ridge comparatively. rapidly; to and fro, so as to avoid excessive heating or melting of the iron at any one point and also to avoid as much as possible the heating of parts of the casting beyond or below the ridge, so that substantially only the. surface of the ridge which later contacts' with the strings is treated. It is very desirable in piano frames to maintain as much of the rib of cast-iron in its original formand quality as possible, in order to utilize the tonal vibration-deadening or inert qualities of cast-iron, but on the other hand, the cutting of the strings into the ordinary cast-iron ridge has the objections above referred to. The satisfactory solution of the problem depends largely upon maintaining as much of the cast-iron constituency as possible, but so treating the very edge of the ridge that cutting in of the strings is prevented.

In this solution of the, problem, the heat is localized to the very edge of the ridge surface of the rib and to aslittle of the metal immediately below this edge surface as is practically possible in order to induce melting of the ridge to a minimum and at thev same time obtain a maximum combination of carbon with the iron.

During this heating operation at such temperature, of about 1200 to 1500" C., the graphitic carbon in the cast iron passes back into solution with the iron to form mostly iron carbides. The

cast iron is heat treated in such a way as to bring the cast iron up to incipient fusion in order to allow the free and graphitic carbon to be redissolved in the iron. The carbon which was originally separate in the casting is re-dissolved, and in this respect the process relates distinctly to cast iron, and differs from the heat treatment of alloys. Upon comparatively sudden cooling of the treated iron, as results from exposing such highly heated localized portions to room temperature, after the electrode is removed, the carbon in the iron is only partially liberated again as graphite, much of the carbon being retained as carbides of iron. Due to the rapid cooling the free or graphitic carbon, re-dissolved in the iron, is held. The transformation of graphitic carbon to these carbides of iron results in an extremely hard ridge edge ca able of resisting the action of the tensioned strings. Piano wire passed thereover under the tension to which it is subjected in actual use dose not imbed itself into the hardened'edge of the ridge.

With the above objects and features in view, a preferred method of the invention will be hereinafter described in connection with the accompanying drawing for illustrative purposes only, in which:--

Figure 1 represents a transverse cross section of a portion of the well-known piano string frame; and

Fig. 2 shows an enlarged view of the ridge of one of these frames, diagrammatically illus-' trated, as treated in accordance with the preferred method of this invention.

As an illustration of a device treated according to my method, there is shown in Fig. 1 a string-frame flange 10 called the front duplex scale. The flange 10 has a rib 11 over which the piano string 12, fastened to the pin 13, passes. The capo tasto bar 15 has also a rib 16 below which the piano string 12 passes. The ridges 16 and 17 of these ribs 11 and 16 are treated as described along the length over which the piano strings pass. After the edges of these ridges of the ribs have been treated according to the preferred method embodied herein, a cross sectional cut thereof would closely resemble Fig. 2 which is enlarged and is diagrammatic only, to show the difference in molecular structure of the respective parts.

The process consists in locally re-heating the ridge of the rib of a cast-iron piano string-frame by bringing the temperature of the ridge to almost the melting point of tl e metal and then allowing this localized heated area to cool within a period of a few minutes, instead of eight hours, as is the case during the original manufacture of the cast-iron piano plate or string-frame.

It is apparent that while the arc is passed rapidly to and fro across the edge of the ridge of the rib, the chemical and physical structure thereof is changing as the temperature of the ridge is increased. As soon as the ridge shows signs of melting, the arc is removed whereupon the rapidcooling of the same causes most of the now combined carbon to be retained by the iron and be disseminated throughout the treated ridge, thus imparting an added hardness thereto.

It will be noted that the arc is applied to a cast i on piece having an exposed angular projecting portion. so that the are simultaneously melts the apex thereof and that the relatively immediate cooling following the removal of the are from any particular part of the apex causes a change in structure of the same to form an apex containing a higher percentage of iron carbides and less free crystals of graphitic carbon. This change in structure of the apex from that of ordinary cast-iron to iron carbides affords resistance to the cutting action of a tensioned wire passed over the apex.

The ridge is modified physically and chemically and thereby a modification of the mechanical properties is obtained, without adversely affecting the properties of the remaining mass of the article. The graphitic carbon of the ridge is redissolved, with the formation of carbides and the retention of such carbides upon cooling, while the mass of cast-iron remains in its graphitic condition. A cast-iron piano plate or frame so constructed is free from distortion in shape or levels, and free from tonal, vibrational energy losses.

The valuable properties of high test grey iron are well recognized, and the same are thus utilized in the ridge of the rib of the frame. On the other hand, the valuable properties of high graphitic cast-iron for piano parts are also recognized, and these are utilized in the portions other than the ridge of the frame. It is apparent in view of the foregoing that a cast-iron piano frame or other piece is selectively heat-treated preferably in one step, whereby one portion of tie iron is adapted to resist the cutting action of tensioned piano strings, while the other remaining portion of the iron is suificiently soft so as not to adversely affect the tonal qualities of the musical tones resulting from the vibration of the strings.

Although an example of the application of the invention to piano parts has been given, it is not desired that the invention be confined thereto, and it will be readily understood by those versed in the art that cast-iron parts for purposes other than for musical instruments such, for example,

as cast-iron metal edges of stair treads, washers,

etc., to resist wear, can be similarly heat-treated.

It may be said that the invention contemplates the conversion of localized portions or selected areas of high graphitic iron castings into high test grey iron, allowing the major portion to remain as high graphitic cast-iron. While the in-- vention as applied in its preferred method has been described, it is to be understood that various changes or modifications of such an application of the invention may be made without departing from the spirit and scope of the invention as defined in the laim.

Furthermore, it is to be noted that the method hereinabove described in connection with the accompanying drawing, is adapted for treating the exposed angular projecting portion of any castiron piece, and is not limited to the method of treating the article hereinabove disclosed and diagrammatically illustrated.

The present application is a division of application, Serial No. 513,671, filed February 2'7, 1931, for Piano string frames. Issued as Patent No. 1,867,788, dated July 19, 1932.

What is claimed is:-

In the method of electrically heat-treating castiron piano plates or string frames having an angular ridge containing a graphitic carbon content bctween two to four percent, the steps which consist in rapidly passing an electric arc of a current density of about 1000 to 2000 amperes per square inch, to and fro across the ridge of the plate at temperatures varying between 1200 to 1500 0., whereby there is a successive intering the arc from said ridge, and finally rapidly air cooling the incipiently fused surface of the ridge whereby a. hardened surface is obtained,-

having a changed physical and chemical property from that of the graphitic carbon castiron the body of the plate, which graphltic carbon structure remains unchanged.

STANLEY WEBER. 

